To reduce the noxious substances comprised in the exhaust gas flow of a diesel engine a denoxing process can be carried out apart from other measures. Such a denoxing process has become known as the SCR method (Selective Catalytic Reduction). In order to be able to carry out the reduction of the nitric oxides on a catalyst suitable for this purpose, a reducing agent must first be added to the exhaust gas flow. The reducing agent employed is ammonia (NH3). As described in DE 200 21 401 U1, the NH3 is drawn in the liquid state from a tank, generally a pressure tank. The withdrawn NH3 is conducted via a supply line from the NH3 tank into the exhaust gas system. Connected in the supply line is a dosing valve actuated by a control mechanism. Different quantities of nitric oxide are present in the exhaust gas flow as a function of the operating state of the diesel engine. In order to be able to carry out maximum reduction of the nitric oxides on the SCR catalyst, a quantity of NH3 matched to the nitric oxide quantity present in the exhaust gas flow must be supplied as the reducing agent. Consequently, it is essential that the supplied NH3 quantity is sufficient to be able to ensure as much as possible the complete reduction of the nitric oxides contained in the exhaust gas flow. On the other hand, for the best possible utilization of resources, the intent is to inject into the exhaust gas flow only that quantity of NH3 which is actually required for carrying out the denoxing process as defined.
The device described in DE 200 21 401 U1 is capable of carrying out the exact dosing of the liquid NH3 present at the dosing valve. However, driving situations may occur, in particular when the tank is largely emptied, in which gaseous NH3 is drawn by the withdrawal nozzle and not liquid NH3. In this case in the supply line an NH3 gas bubble is entrained. However, dosing takes place under the precondition that liquid NH3 is present at the dosing valve. Consequently, in the event gaseous NH3 is dosed without detection, too small a quantity of NH3 is delivered to be capable of completely reducing the nitric oxides contained in the exhaust gas flow at a given operating state. Due to too small an addition of the reducing agent, the nitric oxides are not completely reduced at the SCR catalyst and, therefore, some are discharged to the environment as such.
Building on this discussed prior art, the present invention addresses the problem of further developing a method or a device such that the danger of an erroneous dosing due to the undetected presence of gaseous reducing agent, instead of the liquid one, is avoided.
The problem of gas bubbles at the input side of the dosing valve is solved according to the present invention by cooling the reducing agent supplied to the dosing valve in order to condense the gaseous phases of the reducing agent.